Prostheses of the shoulder are known, which have a first articulation element associated with the meta-epiphysary part of the humerus, and a second articulation element associated with the shoulder.
In particular, in the case of anatomical or normal prostheses, there is a substantially semi-spherical head, mounted on the humerus, which articulates in a mating concave seating, natural or artificial, associated with the glenoid seating of the scapula.
It is also known to use inverse prostheses, which reproduce the gleno-humeral anatomy in an inverse manner, in which there is a humeral cup or insert of the concave type, inserted into the meta-epiphysary part of the humerus, which is able to articulate with a convex element, or glenosphere, fixed to the scapula.
Normally, inverse prostheses are used in cases of serious muscular degeneration of the shoulder, particularly of the rotator cuff muscles.
A fundamental requisite for a prosthesis, whether anatomical or inverse, is that it allows a good articular stability.
Obtaining a good articular stability, apart from the correct positioning of the elements of the prosthesis, which must be identified by the surgeon in an area of very limited size, also depends on the residual rotator cuff muscles and on the articular capsule. In fact, it is necessary to establish, artificially, a dynamic balance with the components of the prosthesis which allows, as much as possible, to recover the normal physiological functions of the shoulder-humerus articulation and which, however, it is difficult to provide.
In both anatomical prostheses and in inverse prostheses, an incorrect positioning of the components of the prosthesis can cause articular instability and contact between the humeral prosthesis and the scapula, with consequent wear of the bone, in the movement of rotation or adduction.
In particular, in anatomical prostheses there may be a risk of wear of the bone tissue due to the rubbing of a part of the prosthesis with the shoulder during the movements of rotation, particularly intra-rotation and extra-rotation.
In inverse prostheses, on the contrary, the typical medialization of the humerus can cause articular instability, which affects 10% of operations, due to the non-appropriate anatomical coupling typical of such prostheses.
In inverse prostheses, we also have the problem of the “scapular notch”, due to the wear of the material which constitutes the humeral cup which rubs against the scapula, mainly at the bottom but also at the rear, and the consequent osteolysis of the glenoid.
Normally, to limit some of these problems, inverse prostheses for the shoulder are positioned with zero retroversion, or a low level of retroversion, of the humerus; the first solution increases the risk of rear scapular notch, the second in any case introduces problems of articular instability and impingement with the shoulder.
All in all, therefore, the positioning of the prostheses, both anatomical and inverse, has the problem that the optimum orientation between the humeral component and the glenoid component is always given, or that this orientation is not known in advance.
This entails a compromise choice between articular stability, to be provided as far as possible according to the dynamic balance described above, and the risk of knocking and rubbing against the shoulder.
This often prevents the prosthesis from being positioned in an optimum manner, and consequently introduces risk factors of articular instability, impediment to certain movements, knocking and rubbing.
Purpose of the present invention is to achieve a humeral prosthesis which, once implanted, has the capacity to center itself according to the dynamic conditions of the shoulder-humerus combination, so as to guarantee articular stability, make movements more simple and physiological, and to improve the overall efficiency and the duration over time of the prosthesis.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.